Current opinion on germ-line mutation rates in the mammal as affected by X-rays and other mutagens is largely based on the 7-locus test of Oak Ridge and Harwell fame. This mouse test has provided factors for the practice of radiation hygiene throughout the world. However, since a sample of seven is a small one, and since there is variation in response among the loci, it is important that other in vivo test systems be employed. Our work with the H-test attempts to fill this need by screening not less than 40 and perhaps several hundred histocompatibility loci. Tests in hybrid progeny (B6 x C)M for effects induced in C spermatogonia, that employ single or split doses (1 day or 7 weeks) and up to 800 rads (20 kV x-rays, 65 rads/min), have thus far indicated no mutagenic effect, although at least 35 mutations have been recovered. The final results will be submitted for publication within six months. We have recently found the spontaneous mutation rate of the C line less than one-third that of the F-1 hybrid. A new X-ray experiment has therefore been initiated with the C line, which may be more suitable to demonstrate small increments in rate. Additional tests will also be done with Mitomycin C.